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This dissertation focuses on bridging the pressure and complexity gap between heterogeneous catalysis and surface science by introducing new instrumental tools that can operate under catalytically relevant conditions (i.e. atmospheric pressures and temperatures higher than room temperature). Thus, some of the few detailed examples of the polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) as an in situ vibrational spectroscopic tool for the elevated-pressure investigation of gas/solid interfaces on planar single crystal model catalyst systems were presented in this work. Furthermore, for the first time in the literature, PM-IRAS technique was applied to study complex multi-component model catalyst structures exhibiting three dimensional morphologies such as metal nanoparticles deposited on a metal-oxide thin film. In order to achieve a molecular understanding of the properties of CO+NO catalytic reaction at elevated temperatures and pressures on Pd based catalysts, adsorption trends of each of the reactant molecules were studied separately on Pd (111). The adsorption properties of CO/Pd (111) and NO/Pd (111) systems both under UHV conditions and at elevated pressures were discussed in a comparative manner to highlight the pressure dependent behavioral differences between these two probe molecules by emphasizing the risks of extrapolating UHV trends to elevated pressure regimes. CO+NO reaction mechanism and kinetics was also studied on Pd (111) by in situ PM-IRAS. Factors affecting the conversion and the selectivity of the Pd (111) model catalyst towards CO+NO reaction at elevated pressures were discussed. Formation of isocyanate containing species?? was also observed and the catalytic implications of this observation was elaborated. Finally, design and characterization of a complex model catalyst composed of supported Pd nano-particles was investigated using CO adsorption at elevated pressures. Catalytic activity of the defect sites on the supported Pd nano-particles towards CO dissociation was demonstrated and compared with Pd (111) to elucidate the significance of the surface morphology of the active sites in a catalytic reaction.